The present invention relates to a method and an apparatus for forming a metal plate into a cup-shaped member that has an inside bearing surface on a thickened wall portion along a circumferential direction thereof. More specifically, the present invention relates to a method and an apparatus for forming a plunger for use in a belt-drive continuously variable transmission (CVT).
As generally known, a belt-drive CVT includes a drive pulley mounted to a drive shaft, a driven pulley mounted to a driven shaft and a belt connecting the drive and driven pulleys. Each of the drive and driven pulleys includes a fixed pulley half fixed to the shaft and a moveable pulley half moveable in an axial direction of the shaft. A pulley fluid pressure chamber is provided on a side of the moveable pulley half. The pulley fluid pressure chamber receives a pressurized working fluid so that the moveable pulley half is axially moved close to or apart from the fixed pulley half. This results in change in contact radii of the belt with respect to the drive pulley and the driven pulley to thereby optimally control a pulley ratio between the drive pulley and the driven pulley.
The pulley fluid pressure chamber is defined by a cylinder disposed on a side of the moveable pulley half and a plunger fixed onto an outer circumferential surface of a rotational shaft, i.e., the drive shaft or the driven shaft. The plunger has a cup-shape and a shaft insertion hole at a bottom thereof through which the rotational shaft extends. The plunger has an annular-shaped spring bearing surface on a locally thickened portion on an inner circumferential periphery of the plunger which has an increased wall thickness as compared to the remaining portion. The spring bearing surface extends in a direction perpendicular to an axial direction of the plunger. A return spring is installed between the plunger and the moveable pulley half. One end of the return spring is mounted onto the spring bearing surface of the plunger, and the other end of the return spring is mounted onto the corresponding spring bearing surface of the moveable pulley half. The moveable pulley half is biased by the return spring toward the fixed pulley half.
The plunger of the conventional art is formed by a press working or a hot forging which are shown in FIGS. 20 and 21, respectively. As illustrated in FIG. 20, the press working includes the steps of forming metal plate 2 into cup-shaped intermediate molded body A that has one open end, forming cup-shaped intermediate molded body A into cup-shaped intermediate molded body B that has a projecting bottom portion on an opposite side of the one open end, forming cup-shaped intermediate molded body B into cup-shaped intermediate molded body C that has a flange along a circumferential periphery of the one open end and a stepped portion on a side of a bottom thereof, forming cup-shaped intermediate molded body C into cup-shaped intermediate molded body D that has a flanged open end and a generally rectangular shape in vertical cross-section, forming cup-shaped intermediate molded body D into cup-shaped intermediate molded body E that has spring bearing surface 4 on a locally thickened portion located in a vertical-middle position on an inner circumference surface of the body, and forming cup-shaped intermediate molded body E into plunger 6 as a final product which has shaft insertion hole 6a at a bottom thereof.
As illustrated in FIG. 21, the hot forging includes the steps of forming cylindrical billet 10 into disk-shaped intermediate molded body F, forming disk-shaped intermediate molded body F into cup-shaped intermediate molded body G that has one open end, forming cup-shaped intermediate molded body G into cup-shaped intermediate molded body H that has a flange along a circumferential periphery of the one open end and spring bearing surface 12 on a thickened wall portion located in a vertical-middle position on an inner circumferential periphery of the body, and forming cup-shaped intermediate molded body H into plunger 14 as a final product which has shaft insertion hole 14a at a bottom thereof.